Fix JsonReaderData buffer refill to respect valid bytes on partial stream reads

[Copilot]

JsonReaderData.ReadBytes treated buffer.Length as valid data during refills, even when _bytesAvailable was smaller after short stream reads. That could copy stale bytes into the next reader window, skew BytesConsumed, and break JSON tokenization.

• Refill logic now uses valid-byte boundaries

  • Leftover detection changed from totalConsumed < buffer.Length to totalConsumed < _bytesAvailable.
  • Leftover slice now uses only unread valid bytes: buffer.AsSpan(totalConsumed, _bytesAvailable - totalConsumed).

• Refill reads now handle short reads correctly

  • Replaced single Read(...) calls with a bounded loop that keeps reading until the target span is filled or EOF.
  • Preserves the intended relationship between _bytesAvailable, buffered content, and Utf8JsonReader final-block behavior.

• Regression coverage for partial-read streams

  • Added a focused test using a stream that intentionally returns small chunks per read.
  • Confirms full parse to EndArray and exact BytesConsumed accounting.

if (_bytesAvailable != 0 && totalConsumed < _bytesAvailable)
{
    var leftover = buffer.AsSpan(totalConsumed, _bytesAvailable - totalConsumed);
    leftover.CopyTo(buffer);
    _bytesAvailable = ReadToEnd(_stream, buffer.AsSpan(leftover.Length)) + leftover.Length;
}

[Copilot]

Pull request overview

Fixes JsonReaderData.ReadBytes refill behavior when a stream returns short/partial reads by ensuring only valid bytes are treated as available, preventing stale buffer data from being reused and skewing BytesConsumed / tokenization.

Changes:

• Adjusted refill “leftover” calculation to use _bytesAvailable (valid bytes) rather than buffer.Length.
• Replaced single Stream.Read(...) calls with a bounded read loop (ReadToEnd) that fills the target span until EOF.
• Added a regression test using a stream that intentionally returns small chunks per read, asserting correct end-to-end parsing and BytesConsumed.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/EFCore/Storage/Json/JsonReaderData.cs	Fixes buffer refill logic to respect valid-byte boundaries and handle partial stream reads via a read-to-fill loop.
test/EFCore.Tests/Storage/Json/JsonReaderDataTest.cs	Adds regression coverage for partial-read streams to validate BytesConsumed correctness through EndArray.

[cincuranet]

Unless totalConsumed can somehow be negative I guess the _bytesAvailable != 0 is reduntant, isn't it?

[cincuranet]

Unless I'm mistaken, this looks like what stream.ReadAtLeast(buffer, buffer.Length, throwOnEndOfStream: false); would do.